Definition and scope of "failed eruptions"
(dcr 4/19/09) It seems to me that we basically have to find a way to define a boundary between so-called “failed eruptions” and episodes of unrest that didn't really have potential to become more than an intrusion or short-lived perturbation of the system. I think that some significant degree of concern about an eruption is a good way to distinguish between these two classes of volcanic unrest. All of the individual episodes that we've included in the list got people concerned to varying degrees, whether it was through elevation of a color code or an increase in monitoring activities. In this light, many of the caldera unrest episodes of the last decades were cause for concern as far as I understand their history, and thus under the “failed eruption” umbrella, at least as a group, per my proposed criterion. Of course, this definition is a moving target as we gain more experience and understanding with caldera unrest and with unrest in general. Thoughts??? (cn 4/17/09) In the case of fresh intrusions at stratocones there's a complete spectrum from little, relatively deep events that never have much chance of going anywhere (unless there's a butterfly effect?), to intrusions that are nip and tuck all the way to shallow depths -- a footrace between forces driving magma toward eruption and acting to stop it. As we were planning the session, I argued that we should restrict ourselves to cases where magma almost erupted, but that's an artificial cutoff, and in my talk I noted that even in the case of fresh intrusions the distinction is really only in degree and result, not in fundamentally different processes. In the case of calderas, the picture gets even cloudier. We know that most disturbances at calderas do NOT lead to eruptions, because there's a big buffering effect. I don't think we'd consider the 1989 swarm under Mammoth Mtn a failed eruption because it never got close to the surface, nor do I consider the 1997 swarm in the south moat a "failed eruption" because it probably didn't have any magma at all, even though at the time at least some of the scientists were nervous. And how about Rabaul back in 1983? Everyone was worried, and magma really did move close to the surface. It eventually erupted in 1994, but in 1984, wasn't it a "failed" eruption? And then there are the open-vent volcanoes -- also driven in many though not all cases by fresh influx of magma at depth -- but with very little "intrusive" groundbreaking above the level of the reservoir. Rather, there are complicated processes in conduits that, I think, involve lava-lamp convection of magma. These volcanoes -- like Mayon, Arenal, Fuego, Shishaldon, etc -- are perpetually on the brink of eruption and small changes in fresh influx rate, in degassing efficiency, in the viscosity of melt and perhaps some chaotic processes too make the difference between eruption and not. Some disturbances lead to eruptions, others don't ... so where would we draw a line? (text of 2008 AGU session proposal) When a volcano becomes restless, one of the primary questions asked of scientists is whether the unrest and underlying processes will lead to a magmatic eruption. "Failed" magmatic eruptions, where magma comes close to erupting but ultimately fails to reach the surface, can have significant negative consequences, particularly if the associated unrest leads to erroneous forecasts. Over the last several decades "failed" magmatic eruptions have been preceded by various combinations of increased degassing and thermal output, phreatic eruptions, shallow earthquake swarms (some with felt and/or low frequency events), and notable ground deformation. Unequivocal cases of failed magmatic eruptiosn include Soufriere de Guadaloupe (1975-76) and Akutan, Alaska (1996). Arguable cases include Mount Baker, Washington (1975), Iliamna, Alaska (1996), Iwate, Japan (1998), Deception Island, Antarctica (1998), Fourpeaked, Alaska (2006), Huila, Colombia (2007), and many others. A few such cases are well-studied, but many are poorly documented in the literature; thus details of these events are often unavailable to scientists for comparison to an ongoing episode of unrest. One of the primary goals of this session is to highlight examples of volcanic unrest that ultimately failed to produce a magmatic eruption. Another goal is to explore possible discriminants that could indicate whether unrest will or won't lead to eruption, along with physical models for failure or arrest of ascending or convecting magma. We encourage contributions from both observational and theoretical perspectives.